There is no correct usage unless it's stipulated in a style manual. And then you must be sure that the style manual is the one you're supposed to be using. If no style manual is prescribed, then find one that you like and use that.
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user21497Nov 5 '12 at 14:44

Stackoverflow content (since you opened the door): Assuming the only variation that needs to be covered is 1/(0 or many) will flunk localization. Localization is hard; no trivial solution will work universally. Mozilla's guidance contains 16 different sets of pluralization rules. developer.mozilla.org/en-US/docs/Localization_and_Plurals
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Dan NeelyNov 5 '12 at 19:14

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@jwpat7 -1: He's obviously using C-like syntax, and you know what he meant: there's no excuse for not having your program automatically pluralize trivial English nouns.
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DaveNov 6 '12 at 4:33

When you write category(s), or category(ies), or whatever, you are not writing an “optional plural” form of the word category. You are writing an abbreviation of the noun phrase “category (or categories)”.

To see this is so, ask yourself how you would pronounce category(s) and convey the intended meaning? You couldn’t. You would have to say something like “category (or categories)”, and that, then, is also how you would write it out.

How you abbreviate that noun phrase is up to you (and your editor or style guide, if you have one).

And notice that this isn’t even a question about English. This question and its answer apply to any written language in which you want to abbreviate a noun phrase containing both the singular and the plural form.